1. Field of the Invention
The present invention is related to a method of cell reselection, and more particularly, to a method of cell reselection with rearranged priorities for cell reselection according to whether D2D communication is supported.
2. Description of the Prior Art
In cellular communication systems, communication devices communicate with each other via abase station, i.e., the base station is an intermediate entity between the communication devices. Such a scenario is inefficient in aspects of coverage area, resource utilization and transmission delay, especially when the communication devices are close to each other and are able to communicate with each other directly. Therefore, device to device (D2D) communication is proposed. In such a situation, two nearby communication devices may communicate (for example, transmitting/receiving packets) with each other directly according to the D2D communication, and the base station does not need to forward the packets transmitted between the communication devices in order to improve a coverage area of the base station, the resource utilization and/or the transmission delay.
According to 3rd Generation Partnership Project (3GPP) standard TS 36.843 section 9.1.2, D2D communication may adopt two modes for resource allocation: Mode 1 and Mode 2. In Mode 1, when two communication devices in coverage of a base station intend to perform D2D communication, the base station is configured to allocate necessary resources to the two communication devices. However, it is not possible to perform D2D communication in Mode 1 when a communication device is out of coverage of the base station. In Mode 2, when two communication devices intend to perform D2D communication, a specific resource is selected from a resource pool to transmit data. Therefore, Mode 2 is feasible even when a communication device is out of coverage or at the edge of coverage of the base station, and a communication device entering idle mode would mainly support Mode 2.
However, a communication device has no idea whether the network node supports D2D communication and whether Mode 2 or Mode 1 of D2D communication is supported. Furthermore, the magnitudes of CellReselectionPriority defined in TS 36.331 is irrelevant to whether the network node supports D2D communication or not; therefore, neighbor cells with highest priority or frequencies with highest priority on the basis of CellReselectionPriority may not support D2D communication. When the communication device performs cell reselection according to CellReselectionPriority broadcasted by the network node, the communication device may reselect to a neighbor cell which does not support D2D communication and is thus forced to terminate the ongoing D2D communication. There is a need for an improved method of cell reselection.